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Background[ edit ] There were several efforts to improve oscillators in the s. Linearity was recognized as important. The "resistance-stabilized oscillator" had an adjustable feedback resistor; that resistor would be set so the oscillator just started thus setting the loop gain to just over unity.
I would report on some applications I had thought up on negative feedback, and the boys would read recent articles and report to each other on current developments.
This seminar was just well started when a paper came out that looked interesting to me. It was by a man from General Radio and dealt with a fixed-frequency audio oscillator in which the frequency was controlled by a resistance-capacitance network, and was changed by means of push-buttons. Oscillations were obtained by an ingenious application of negative feedback.
The oscillator was demonstrated in Portland. The first sale was in January The conventional oscillator circuit is designed so that it will start oscillating "start up" and that its amplitude will be controlled. The oscillator at the right uses diodes to add a controlled compression to the amplifier output.
The linear oscillator can support any amplitude. In practice, the loop gain is initially larger than unity. Random noise is present in all circuits, and some of that noise will be near the desired frequency. A loop gain greater than one allows the amplitude of frequency to increase exponentially each time around the loop.
With a loop gain greater than one, the oscillator will start. Ideally, the loop gain needs to be just a little bigger than one, but in practice, it is often significantly greater than one. A larger loop gain makes the oscillator start quickly. A large loop gain also compensates for gain variations with temperature and the desired frequency of a tunable oscillator. For the oscillator to start, the loop gain must be greater than one under all possible conditions.
A loop gain greater than one has a down side. In theory, the oscillator amplitude will increase without limit. In practice, the amplitude will increase until the output runs into some limiting factor such as the power supply voltage the amplifier output runs into the supply rails or the amplifier output current limits. The limiting reduces the effective gain of the amplifier the effect is called gain compression. In a stable oscillator, the average loop gain will be one.
Although the limiting action stabilizes the output voltage, it has two significant effects: it introduces harmonic distortion and it affects the frequency stability of the oscillator. The amount of distortion is related to the extra loop gain used for startup. That means more distortion. The amount of distortion is also related to final amplitude of the oscillation.
The nonlinear transfer function can be expressed as a Taylor series. For small amplitudes, the higher order terms have little effect. For larger amplitudes, the nonlinearity is pronounced.
Unmarked capacitors have enough capacitance to be considered short circuits at signal frequency. Unmarked resistors and inductor are considered to be appropriate values for biasing and loading the vacuum tube. Node labels in this figure are not present in the publication.
Meacham disclosed the bridge oscillator circuit shown to the right in The circuit was described as having very high frequency stability and very pure sinusoidal output. The crystal, Z4, operates in series resonance. As such it minimizes the negative feedback at resonance. The particular crystal exhibited a real resistance of ohms at resonance. At frequencies below resonance, the crystal is capacitive and the gain of the negative feedback branch has a negative phase shift.
At frequencies above resonance, the crystal is inductive and the gain of the negative feedback branch has a positive phase shift. The phase shift goes through zero at the resonant frequency. As the lamp heats up, it decreases the positive feedback. At any frequency different from the resonant frequency by more than a small multiple of the bandwidth of the crystal, the negative feedback branch dominates the loop gain and there can be no self-sustaining oscillation except within the narrow bandwidth of the crystal.
Unmarked resistors are considered to be appropriate values for biasing and loading the vacuum tubes. Node labels and reference designators in this figure are not the same as used in the patent.
William R. At the oscillating frequency, the bridge is almost balanced and has very small transfer ratio. The loop gain is a product of the very high amplifier gain and the very low bridge ratio. R1, R2, C1 and C2 form a bandpass filter which is connected to provide positive feedback at the frequency of oscillation. Rb self heats and increases the negative feedback which reduces the amplifier gain until the point is reached that there is just enough gain to sustain sinusoidal oscillation without over driving the amplifier.
When the circuit is first energized, the lamp is cold and the gain of the circuit is greater than 3 which ensures start up. The dc bias current of vacuum tube V1 also flows through the lamp. It has the ease of handling of a beat-frequency oscillator and yet few of its disadvantages.
In the first place the frequency stability at low frequencies is much better than is possible with the beat-frequency type. There need be no critical placements of parts to insure small temperature changes, nor carefully designed detector circuits to prevent interlocking of oscillators. As a result of this, the overall weight of the oscillator may be kept at a minimum. An oscillator of this type, including a 1 watt amplifier and power supply, weighed only 18 pounds, in contrast to 93 pounds for the General Radio beat-frequency oscillator of comparable performance.
The distortion and constancy of output compare favorably with the best beat-frequency oscillators now available. Lastly, an oscillator of this type can be laid out and constructed on the same basis as a commercial broadcast receiver, but with fewer adjustments to make. It thus combines quality of performance with cheapness of cost to give an ideal laboratory oscillator. Main article: Wien bridge Bridge circuits were a common way of measuring component values by comparing them to known values.
Often an unknown component would be put in one arm of a bridge, and then the bridge would be nulled by adjusting the other arms or changing the frequency of the voltage source see, for example, the Wheatstone bridge. The Wien bridge is one of many common bridges. The Wien bridge does not require equal values of R or C. At some intermediate frequency, the phase shift will be zero. At that frequency the ratio of Z1 to Z2 will be purely real zero imaginary part.
If the ratio of Rb to Rf is adjusted to the same ratio, then the bridge is balanced and the circuit can sustain oscillation. There will always be a frequency at which the total phase shift of each branch of the bridge will be equal.
Operation Edit This article needs attention from an expert on the subject. The specific problem is: The description is confused about the operation of the circuit, and the diagrams do not show crucial phase lag components or the need for a high gain amplifier. The output of the amplifier should show a series R driving the first shunt C. That lag network is beyond cutoff so it has almost a 90 degree phase shift and significant attenuation. The crystal is operated near series resonance, so it looks like a resistor, and that resistor forms a second lag network for almost another 90 degree phase shift. The inverting amplifier supplies degree phase shift plus a little more because it is not infinite bandwidth. See, for example, Matthys pages 45—
CRYSTAL OSCILLATOR CIRCUITS MATTHYS PDF
Not entirely safe, but never bored Thar be dragons! There are other versions of this kit in other colors that use the same circuit. This post explains why. It is billed as a DC MHz frequency counter and crystal checker. The idea of a crystal oscillator coupled with a frequency counter is a good one—plug in an old crystal you got in the flea market and it will not only tell you if it is oscillating, but it will also tell you at what frequency it is oscillating. Some might question why you need to measure the frequency when its written on the case, but this is often not true when it comes to old amateur crystal holders or WWII surplus that was later modified; labels fall off, cases are switched, quartz blanks are reground, etc.